Extending tree diversity comparison to Southern Hemisphere

This is a continuation of my comparison of the tree species richness in temperate regions (viewtopic.php?f=144&t=6804). From Larry’s message (#7) I got the idea of extending the comparison to the southern temperate world. However, I developed my method for the northern temperate regions and it is not easily applicable to the southern temperate regions. The main reason is that in the south only a few genera are shared with the northern regions and even with other southern regions. If a shrub genus is erroneously included or a tree genus omitted, it immediately distorts the comparison. In the north, the effect of such an error is much smaller because the genera are largely the same. Consequently, if southern regions are included, the comparability is lower than between the northern sub-regions. I return to the generic similarities between the regions below. Here I describe my methods and definitions only if they differ from those I described in my northern comparison (see the link above).

Delimiting the study regions

I had difficulties here, too. Eastern South America is treeless around the 13.5°C point, in Africa and western Australia no coastal place is so cool and New Zealand is slightly too small and the island effect lowers the species richness. Thus, only two sub-regions were possible to delimit.

Western South America: Central Chile

In my source for the climate data, there is no point close to 13.5°C along the coast, the closest being La Serena (14.5°C) and Concepción (12.4°C). I decided to choose the halfway point between them at the coast as my starting point. This point is located west from the capital Santiago. Then I encountered problems. Entire Chile is too large and extends too far north and south. One possibility would be to continue to Argentina and even to Uruguay and select regions of all 2-3 countries. However, the climate east of the Andes is much warmer and drier, and much of the area is not forested. On the east coast, the 13.5°C point is far south of the western point. Thus, I decided to include the Chilean regions closest to the starting point until the area requirement was met. The following regions were included: Atacama, Coquimbo, Valparaíso, Metropolitana de Santiago, O'Higgins, Maule, Bío Bío, La Araucanía, Los Ríos and Los Lagos. The area of the sub-region is 313,776 km2.

Location of the sub-region (in red).

The climate and vegetation parallel those of California extending from desert through mediterranean type to temperate rainforest and high mountains. The climate is very oceanic. In the southern parts of the sub-region precipitation is abundant throughout the year and the vegetation dominated by evergreen angiosperms.

I mostly used http://www.chileflora.com for the genus information and species distribution data. Wikipedia helped to decide whether a given genus is a tree genus.

Southeastern Australia

The sub-region delimitation was problematic again. The southernmost point of Victoria, Tidal River, is still slightly too warm (14°C). Going south, Flinders Island between Victoria and Tasmania has 13.4°C, and Devonport in northern Tasmania 12.7°C. I decided to make an exception to my rule not to accept islands as major parts of sub-regions and to form the sub-region from Victoria and Tasmania. Reasons: The greater part of the sub-region still lies on the mainland, Tasmania has been connected to the mainland through most of the Quaternary, and almost two thirds of the Tasmanian tree species are shared with Victoria. The area of the sub-region is 306,030 km2.

Location of the sub-region (in red).

Australia_map.jpg (36.62 KiB) Viewed 943 times

Rainfall is relatively evenly distributed but winter rainfall predominates. Northern Victoria is quite dry but moisture increases to the south. In Tasmania, westerly winds, “the Roaring Forties”, predominate bringing high rainfall to the western part. Western Tasmania has the smallest temperature amplitude of all our sub-regions. The highest peak, Mount Kosciuszko in Victoria, is 2228 m. In Australia, eucalypts (genera Eucalyptus, Corymbia and Angophora) have evolved to occupy the niche of northern fire-dependent conifers like pines and Douglas-firs, and they have succeeded superbly: even in western Tasmania there are few areas completely devoid of eucalypts. Almost the whole tree flora is evergreen.

I took the genera from “Native Trees and Shrubs of South-Eastern Australia” by Costermans and “Native Trees of Tasmania” by Kirkpatrick & Backhouse. Wikipedia, NSW Flora Online (http://plantnet.rbgsyd.nsw.gov.au) and “Eucalypts of Victoria and Tasmania” by Nicolle helped in deciding whether a given genus is a tree genus. For species distribution I used A Census of the Vascular Plants of Victoria (http://www.rbg.vic.gov.au/viclist) and the Tasmanian vascular plant census (http://www.tmag.tas.gov.au/collections_ ... blications). I still had a major problem. The tree flora of the sub-region is dominated by only two genera: Eucalyptus and Acacia. The problem was the latter: there are 111 Acacia species in the sub-region, about half of them being trees. Should I include the genus? The decision would have a disproportionate influence on the results. Therefore I decided to change my method in the case of these two genera and include only the tree acacias and eucalypts (only two eucalypt species do not meet my tree definition). I did not find any description for 14 Acacia species, so I included the same proportion of them (6 species) as was the proportion of trees for the other Acacia species (45 out of 97). There are four tree ferns in the sub-region but according to my research most Cyathea species are not tree-sized; I then decided to omit the other genus, Dicksonia, too, because only one fern species would give misleading information in my opinion.

Results

Species numbers by genera in 2 sub-regions are listed in the Excel file below.

For the comparison of the southern and northern sub-regions, one further adjustment was needed: I had previously omitted Sorbus, Crataegus and Malus because they contain a lot of apomictic microspecies. I know that there are 5 non-apomictic Sorbus, 3 Crataegus and 1 Malus in France. In the summary below, I have added 9 angiosperms for France and for the other northern sub-regions a number of angiosperms so that the proportions of angiosperm numbers between the northern sub-regions remain the same.

Interestingly, Central Chile and southeastern Australia have similar species richness except that the latter has additionally an enormous diversity of Eucalyptus and Acacia. No genus in any other sub-region has as many species as either of these two genera in the Australian sub-region.

Finally, let’s compare similarities in the floristic compositions of the sub-regions. In the table below I have tabulated all the sub-region pairs. The figures show the percentage of the species for which there are con-generic species in the pair sub-region. I have grouped the similarities to three levels: very strong similarities (88–96%, dark orange), strong similarities (47–73%, light orange) and no or weak similarities (0–9%, white).

It is easy to see that the sub-regions fall into three groups: 1. Northern Hemisphere, 2. Central Chile, and 3. Southeast Australia. The Northern Hemisphere breaks down further into four sub-groups: 1.a Western Eurasia, 1.b East China, 1.c Eastern US, and 1.d California. It is not difficult to find the reasons for this grouping. All the northern sub-regions are in the same group as they were still connected 5 millions years ago, but Australia began to separate from the South America – Antarctica block as early as about 80 millions years ago. The tropical zone effectively isolates the northern and southern temperate regions. France, the Balkans and the Caucasus are in the same sub-group as they are located close to each others in the same geographical area.